A LoRa link budget tells you whether two radios can hear each other — and by how much. It is just addition and subtraction in decibels, but it explains everything about LoRa, Meshtastic and LoRaWAN range: why antenna height matters, why slower spreading factors reach further, and why RSSI alone does not tell you if a link is healthy. This article walks through the LoRa link budget step by step, with real numbers for the SX1262 chips used in most modern boards.
The link budget formula
Received power (dBm) = TX power + TX antenna gain − TX cable loss
− path loss + RX antenna gain − RX cable loss
Link margin (dB) = Received power − receiver sensitivityIf the margin is positive, the link works. In practice you want at least 10 dB of margin, because fading, rain, foliage and people walking past change the signal constantly.
Free-space path loss
In free space the loss grows with distance and frequency:
FSPL (dB) = 20·log10(d, km) + 20·log10(f, MHz) + 32.44At 915 MHz, 1 km costs about 91.7 dB, 10 km about 111.7 dB and 100 km about 131.7 dB. Every doubling of distance adds 6 dB. In real terrain, with obstacles and ground reflections, loss grows faster — often as distance to the power of 3 to 4 instead of 2.
LoRa sensitivity by spreading factor
LoRa’s trick is its very low sensitivity. Typical SX1262 values:
| Spreading factor | 125 kHz bandwidth | 250 kHz bandwidth |
|---|---|---|
| SF7 | about −124 dBm | about −121 dBm |
| SF9 | about −130 dBm | about −127 dBm |
| SF11 | about −135 dBm | about −132 dBm |
| SF12 | about −137 dBm | about −134 dBm |
Each step up in spreading factor gains about 2.5–3 dB and doubles the airtime. Halving the bandwidth gains another 3 dB.
Worked example
Two Meshtastic nodes on Long Fast (SF11, 250 kHz), 22 dBm TX, 2 dBi antennas, 1 dB cable loss in total:
- EIRP and RX gain: 22 + 2 + 2 − 1 = 25 dB
- Sensitivity: about −132 dBm
- Total budget: 25 − (−132) = 157 dB
- Keep 10 dB margin, so up to 147 dB of path loss is acceptable.
In free space that is hundreds of kilometers. Over real ground it is a few kilometers between handhelds and tens of kilometers from a hilltop. Try your own numbers in the Meshtastic range calculator.
RSSI, SNR and what they really mean
- RSSI is the received power in dBm. −90 dBm is strong, −120 dBm is weak but often fine for LoRa.
- SNR is how far the signal is above the noise. LoRa can decode signals below the noise floor: down to about −7.5 dB at SF7 and −20 dB at SF12.
A link with RSSI −115 dBm and SNR +5 dB is healthy. A link with RSSI −95 dBm and SNR −10 dB is in trouble: something nearby is raising the noise floor. Our guide to dBm, dB, dBi and RSSI explains these units in more detail.
Where the budget is lost in practice
- Antenna mismatch. SWR 2.5 loses about 0.9 dB, but a detuned stock antenna often loses 3–6 dB through poor efficiency. Test it.
- Cable. Thin RG-174 loses about 1 dB per meter at 915 MHz.
- Obstacles. A single building wall: 5–15 dB. A wooded hill: much more.
- Noise. Switching power supplies and USB hubs near the node raise the noise floor and kill SNR. A spectrum analyzer shows it immediately.
Link budget with terrain, Fresnel zone and margin
Our free terrain calculator uses real elevation data, Earth curvature and ITU-R P.526 obstruction loss to show whether your LoRa link will work.
Calculate a real linkFAQ
How much link margin do I need for LoRa?
At least 10 dB for a reliable link, 15–20 dB for critical infrastructure such as a router node that must never drop out.
Why does my RSSI look good but messages fail?
Look at SNR. If it is near the decoding limit, local interference is the problem, not distance. Move the node away from electronics or add a filter.
Does LoRaWAN use the same link budget?
Yes. The gateway usually has a better antenna and height, and adaptive data rate picks the spreading factor that keeps a sensible margin.
Sources: Semtech: LoRa modulation basics and Free-space path loss (Wikipedia).